1. Field of the Invention
The present invention relates to a method of producing a bismuth layered compound for use in an electronic device such as a ferroelectric memory, for example.
2. Description of the Related Art
Bismuth layered compounds including bismuth superconducting oxides having a critical temperature of 110 K and ferroelectric memory materials play a very important role in industrial applications. For applying such bismuth layered compounds to electronic devices, it is necessary to develop a thin-film fabrication process for those bismuth layered compounds.
The structure of a bismuth layered compound, e.g., Bi.sub.2 PbNb.sub.2 O.sub.9, is of a pseudo tetragonal system having a columnar shape extending in the direction of a c-axis. It has a repetitive structure comprising layers of a bismuth oxide and oxides of other elements which are laminated in a certain sequence (see G. A. SMOLENSKII et al., SOVIET PHYSICS--SOLID STATE, p. 651-655 (1961) and E. C. SUBBARAO, J. Phys. Chem. Solids Pergamon Press. Vol. 23, p. 665-676 (1972)).
In the repetitive structure, the number of bismuth oxide layers in one unit structure and the length of the unit structure vary from bismuth layered compound to bismuth layered compound.
Attempts to apply such bismuth layered compounds to electronic devices have been made in the art. According to one of the efforts, a thin film of a bismuth layered compound which exhibits good ferroelectric properties is produced by a spin coating process such as a MOD (Metal Organic Deposition) process.
However, the spin coating process fails to meet requirements for clean environments that are to be established in the actual semiconductor fabrication process.
For the above reasons, there has been a need for a new thin-film fabrication process for manufacturing a thin film of a bismuth layered compound. To make a thin film of an oxide, however, it is difficult to rely on oxidation carried out by an ultrahigh vacuum process such as molecular beam epitaxy or laser ablation. The application of the MOCVD (Metal Organic Chemical Vapor Deposition) process which is widely used for semiconductor fabrication has encountered obstacles because a hydrogen gas cannot be used as a carrier and a good source material is not available.